Papillomaviruses infect a wide variety of different species of animals including humans. Infection is typically characterized by the induction of benign epithelial and fibro-epithelial tumors, or warts at the site of infection. Each species of vertebrate is infected by a species-specific set of papillomavirus, itself comprising several different papillomavirus types. For example, more than sixty different human papillomavirus (HPV) genotypes have been isolated. Papillomaviruses are highly species-specific infective agents. For example, canine and rabbit papillomaviruses cannot induce papillomas in heterologous species such as humans. Neutralizing immunity to infection against one papillomavirus type generally does not confer immunity against another type, even when the types infect a homologous species.
In humans, papillomaviruses cause genital warts, a prevalent sexually-transmitted disease. HPV types 6 and 11 are most commonly associated with benign genital warts condylomata acuminata. Genital warts are very common, and subclinical or inapparent HPV infection is even more common than clinical infection. While most HPV-induced lesions are benign, lesions arising from certain papillomavirus types, e.g., HPV-16 and HPV-18, can undergo malignant progression. Moreover, infection by one of the malignancy-associated papillomavirus types is considered to be a significant risk factor in the development of cervical cancer, the second most common cancer in women worldwide. Of the HPV genotypes involved in cervical cancer, HPV-16 is the most common, being found in about 50% of cervical cancers. The prevalence of HPV-18 ranges from approximately 8-31% depending on the geographical location, and in most areas worldwide, HPV-45 is the third most frequent, oncogenic HPV type (Bosch, F. X., et al. (1995, J. Natl. Cancer Inst. 87: 796-802).
In view of the significant health risks posed by papillomavirus infection generally, and human papillomavirus infection in particular, various groups have reported the development of recombinant papillomavirus antigens and their use as diagnostic agents and as prophylactic vaccines. In general, such research has been focused toward producing prophylactic vaccines containing the major capsid protein (L1) alone or in combination with the minor capsid protein (L2). For example, Ghim et al, Virology, 190:548-552 (1992), reported the expression of HPV-1 L1 protein, using vaccina expression in Cos cells, which displayed conformational epitopes and the use thereof as a vaccine or for serological typing or detection. This work is also the basis of a patent application, U.S. Ser. No. 07/903,109, filed Jun. 25, 1992 (abandoned in favor of U.S. Ser. No. 08/216,506, filed on Mar. 22, 1994), which has been licensed by the assignee of this application. Also, Suzich et al, Proc. Natl. Acad. Sci., U.S.A., 92:11553-11557 (1995), report that the immunization of canines with a recombinant canine oral papillomavirus (COPV) expressed in a baculovirus/insect cell system completely prevented the development of viral mucosal papillomas. These results are important given the significant similarities between many HPVs and COPV. For example, COPV, similar to HPVs associated with anogenital and genital cancer, infects and induces lesions at a mucosal site. Also, the L1 sequences of COPV shares structural similarities to HPV L1 sequences. Given these similarities, the COPV/beagle model is useful for investigation of L1 protein-containing vaccines, e.g., investigation of the protective immune response, protection from natural infection and optimization of vaccination protocols. (Id.)
Also, a research group from the University of Rochester reported the production of human papillomavirus major capsid protein (L1) and virus-like particles using a baculovirus/insect cell expression system (Rose et al, University of Rochester, WO 94/20137, published on Sep. 15, 1994). In particular, they reported the expression of the L1 major capsid protein of HPV-6 and HPV-11 and the production of HPV-6, HPV-11, HPV-16 and HPV-18 virus-like particles.
Further, a University of Queensland research group also purportedly disclosed the recombinant manufacture of papillomavirus L1 and/or L2 proteins and virus-like particles as well as their potential use as vaccines (Frazer et al, WO 93/02189, published Feb. 4, 1993).
Still further, a United States government research group reported recombinant papillomavirus capsid proteins purportedly capable of self-assembly into capsomere structures and viral capsids that comprise conformational antigenic epitopes (U.S. Pat. No. 5,437,951, Lowy et al, issued Aug. 1, 1995). The claims of this patent are directed to a specific HPV-16 DNA sequence which encodes an L1 protein capable of self-assembly and use thereof to express recombinant HPV-16 capsids containing said HPV-16 L1 protein.
With respect to HPV capsid protein containing vaccines, it is now widely accepted by those skilled in the art that a necessary prerequisite of an efficacious HPV L1 major capsid protein-based vaccine is that the L1 protein present conformational epitopes expressed by native human papillomavirus major capsid proteins (see, e.g., Hines et al, Gynecologic Oncology, 53:13-20 (1994); Suzich et al, Proc. Natl. Acad. Sci., U.S.A., 92:11553-11557 (1995)).
Both non-particle and particle recombinant HPV L1 proteins that present native conformational HPV L1 epitopes have been reported in the literature. It is known that L1 is stable in several oligomeric configurations, e.g., (i) capsomeres which comprise pentamers of the L1 protein and (ii) capsids which are constituted of seventy-two capsomeres in a T=7 icosahedron structure. Also, it is known that the L1 protein, when expressed in eukaryotic cells by itself, or in combination with L2, is capable of efficient self-assembly into capsid-like structures generally referred to as virus-like particles (VLPs).
VLPs have been reported to be morphologically and antigenically similar to authentic virions. Also, immunization with VLPs has been reported to elicit the production of virus-neutralizing antibodies. For example, results with a variety of animal papillomaviruses (canine oral papillomavirus and bovine papillomavirus-4) have suggested that immunization with VLPs results in protection against subsequent papillomavirus infection. Consequently, VLPs composed of HPV L1 proteins have been proposed as vaccines for preventing diseases associated with human papillomavirus infections.
Specifically, it has been reported that the L1 protein can assemble into VLPs when expressed using recombinant baculovirus and vaccinia virus vectors and in recombinant yeast (Hagensee et al, J. Virol., 68:4503-4505 (1994); Hofmann et al, Virology, 209:506-518 (1995); Kirnbauer et al, Proc. Natl. Acad. Sci. USA, 89:12180-12184 (1992); Kirnbauer et al, J Virol., 67:6929-6936 (1993); Rose et al, J Virol., 67:1936-1944 (1993); Sasagawa et al, Virology, 206:126-135 (1995); Suzich et al, Proc. Natl. Acad. Sci. USA, 92:11553-11557 (1995); Volpers et al, Virology, 200:504-512 (1994); Zhou et al, J. Virol., 68:619-625 (1994)).
Most previous recombinant L1 preparations isolated from eukaryotic cells have resulted in a variable population of VLPs approaching 55 nm in diameter, which are similar in appearance to intact virions. However, VLP assembly is somewhat sensitive to cell type. For example, L1 expressed in Escherichia coli is expressed largely in the form of capsomeres or smaller, with few or no capsids apparent either in the cell or upon purification (Rose et al, J. Virol., 67:1936-1944 (1993); L1 et al, J. Virol., 71:2988-2995 (1997)). Similar results are observed when the polyoma virus VP1 protein is expressed in E. coli (Salunke et al, Biophys. J., 56:887-900 (1989)).
While eukaryotic cells have been focused on for the production of papillomavirus capsid proteins, and that of other viruses, because of their reported capability to express these proteins and VLPs such that they present appropriate conformational, neutralizing epitopes, there have been some reports of the use of bacteria for the expression of papillomavirus capsid proteins, both in fused and non-fused form, as well as for the manufacture of capsomeres.
For example, it was disclosed in PCT/US98113799, entitled “Homogeneous Human Papillomavirus Capsomere Containing Compositions, Methods for Manufacture, and Use Thereof as Diagnostic, Prophylactic or Therapeutic Agents”, that capsomeres expressed in E. coli were capable of generating neutralizing antisera in rabbits that prevented papillomavirus infection in a model tissue culture assay. These capsomeres were expressed as non-fusion proteins. Therefore, the expressed L1 proteins did not include any non-PV coding sequences.
Also, L1 et al, J. Virol., 71:2987 (1997) reported the expression of a full length, non-fusion HPV 11 L1 protein in E. coli that presented conformational epitopes. However, expression levels were relatively low, and the purification procedure required to isolate the expression product was quite laborious.
Further, Lin et al, Viriology, 187:612-619 (1992) reported the expression of CRPV trpE-L1 fusion proteins in E. coli and the use of the resultant expression product to induce antisera in rabbits. The fusion protein was expressed in insoluble, refiactile body form, and the insoluble fractions containing the trpE fusion proteins were characterized by SDS-PAGE and the resultant “crude fusion protein” used to immunize rabbits together with an immune adjuvant (MPL and TDM and CWS emulsion) (Ribi Immunochemical Research Inc.(adjuvant)). However, such an impure protein extract would likely be unsuitable for use as a vaccine because of potential endotoxin contamination. Additionally, Lin et al, in J. Virol., 67(7):4154-4162 (1993) reported the expression of CRPV L1 protein in E. coli as a TrpE fusion protein. They report therein the identification of neutralizing epitopes and further disclose that a successful papillomavirus vaccine must be based on immunization with full-length, native L1 and that smaller peptides containing major linear epitopes is not feasible.
Still further, Zhang et al, Virology, 243:4236-431 (1996) reported the expression of HPV16 L1 proteins in E. coli, wherein the L1 sequence was fused at its amino terminus to a 24-amino acid leader sequence, pelB, and the carboxy terminus to six histidine residues (His tag). However, disadvantageously, the bacterial expressed L1 protein was in the form of insoluble aggregates (inclusion bodies) which were expressed at a yield of more than 10% total cell proteins. The insoluble proteins, when isolated with 8M urea and purified by chromatographic separation, after removal of the urea, spontaneously reunited and assembled into polymorphologic aggregations in vitro which included structures resembling native empty capsids as well as incompletely formed capsids. The correctly folded VLPs were purified by sucrose gradient sedimentation, and were recognized by an HPV16 type-specific, conformational monoclonal antibody in an ELISA. However, the purification procedure was quite laborious, which is disadvantageous in the context of vaccine preparation, wherein high protein yields at reasonable cost are necessary.